Sigma Notation Help

**nightrider456** · February 26th 2010, 10:22 PM

We've had a couple of classes on the topic with geometric sequences and arithmetic series as well. I've been having a propblem finding the values to use in the sum of the geometric sequence formula.
$ tn=\frac{ar^n-1}{r-1} $
8
$ \sum(1/3)^(k-9) $
k=4
I`m having trouble getting the a and r values,is there any general rules you can use when looking for them in sigma notation

**Prove It** · February 26th 2010, 11:15 PM

Originally Posted by nightrider456

We've had a couple of classes on the topic with geometric sequences and arithmetic series as well. I've been having a propblem finding the values to use in the sum of the geometric sequence formula.
$ tn=\frac{ar^n-1}{r-1} $
8
$ \sum(1/3)^(k-9) $
k=4
I`m having trouble getting the a and r values,is there any general rules you can use when looking for them in sigma notation

I take it that this is actually

$\sum_{k = 4}^8 \left(\frac{1}{3}\right)^{k - 9}$ .

Why not try writing out the first few terms?

$\sum_{k = 4}^8 \left(\frac{1}{3}\right)^{k - 9} = \left(\frac{1}{3}\right)^{-5} + \left(\frac{1}{3}\right)^{-4} + \left(\frac{1}{3}\right)^{-3} + \dots$

$= 243 + \frac{1}{3}\cdot 243 + \frac{1}{3}\cdot \frac{1}{3} \cdot 243 + \dots$ .

Can you see what $a$ and $r$ are now?

Thread: Sigma Notation Help

LinkBack

Thread Tools

Display

Sigma Notation Help

Similar Math Help Forum Discussions

sigma notation

Sigma notation help

sigma notation

Sigma Notation

Sigma notation

Search Tags